CONCLUSIONS
Strength gains are specific: to the muscle action you use. So eccentric training produces greater gains in eccentric strength than in concentric strength (and vice versa).
Specific strength requirement: knee flexor (and possibly also knee extensor) eccentric strength; hip flexor and hip extensor concentric strength.
Since strength is specific, then: strength training with eccentric muscle actions or accentuated eccentric loading for the knee flexors (and possibly the knee extensors) should lead to superior gains in sprinting performance compared to strength training with similar absolute load in eccentric and concentric phases.